1. Field of the Invention
The present invention relates to a method and apparatus for curvature-resistant micro-mirror structures to reduce light beam loss due to mirror curvature in free-space micro-machined optical switches, and more particularly, to a method and apparatus for curvature-resistant micro-mirror structures having an enhanced construction which prevents mirror curvature due to thermal and residual stresses associated with surface micro-machining processes.
2. Description of the Art
Due to very rapid increases in core-transport network demand (e.g., in the bit rates of individual services and in the number of wavelength channels being built into Wavelength Division Multiplexing (WDM) transport systems), fiber-optic switches with large port-count have quickly emerged as perhaps the most important, yet unrealized, technological need in future high-capacity light-wave networks. These fiber-optic network elements will be chiefly used for network restoration to begin with, with substantial provisioning value likely emerging thereafter.
The optical switching technologies advanced so far offer the potential advantages of bit-rate transparency, low power consumption, small volume, and low cost. Nevertheless, the requirements in port count (on the order of 512.times.512 and larger) and in loss budget represent deep challenges that have not yet been met by any current photonic switching technology. Although conventional mechanical switches can achieve high optical quality, they are large in size and mass, and are thus relatively slow in switching speed. On the other hand, guided-wave solid-state switches, though compact, generally have high loss and high crosstalk. The inherent disadvantages of these technologies thus appear to limit their expandability to the port counts mentioned above.
By contrast, free-space micro-machined optical-switching technology holds particular appeal in this application because it combines the advantages of free-space interconnection (i.e., low loss and high optical quality) with those of monolithic integrated optics, namely, compactness. Various small-scale (2.times.2) micro-machined switches utilizing sliding micro-mirrors have been demonstrated. In addition, collimating optics and rotating micro-mirrors have also been proposed as a means of achieving high-density optical switches. Given the fertility that the field of micro-optical systems is beginning to show, and the considerable variety of switching devices that has already emerged, it is likely that diverse applications will be best suited to diverse switching technologies. However, for the application of restoration and provisioning in core-transport light-wave networks, free-space micro-machined optical switches (FS-MOS) with free-rotating hinged micro-mirrors are particularly attractive. Attractive because such applications do not require frequent switching, but do require very high reliability even for switch mirrors that remain in one switching state for extended periods on the order of years. Furthermore, the sub-millisecond switching times exhibited by FS-MOS devices are well-matched to the needs of restoration and provisioning in core-transport light-wave communications networks.
However, like other optical cross-connect technologies, free-space micro-machined optical switches face the substantial challenge of achieving increased port-count while living within a specified loss budget that will in practice be fixed by adjacent transponders. Unfortunately, one significant contributor to light beam loss is the curvature of the micro-mirrors in these systems.